Add chruby and chruby-auto plugins

  * chruby.bash loads chruby
  * chruby-auto.bash loads chruby and enables auto-switching
  * add chruby_version_prompt() function for displaying ruby version
  * inspired by https://gist.github.com/rssvihla/6153455
Merge pull request #233 from aramprice/master

Add chruby and chruby-auto plugins